Itinerant spin and valley ferromagnetism in rhombohedral trilayer graphene (Part I)

In the following two talks we report the observation of ferromagnetism in rhombohedral (ABC-stacked) trilayer graphene. Under an applied electrical displacement field, rhombohedral trilayer graphene is a semiconductor with an unusually flat band minimum. Here we show that the resulting high density of states near the band edges leads to an instability towards ferromagnetic states in which one or more of the spin and valley symmetries are broken. Using capacitance measurements to probe the inverse compressibility as a function of carrier density and perpendicular electric field, we find a series of gate tuned first order transitions---demarcated by sharp negative compressibility features and change in the degeneracy of low-magnetic field quantum oscillations. In the first of the two talks, I will describe our fabrication process for building high mobility multi-layer heterostructures while preserving the metastable rhombohedral stacking order. I will also describe our capacitance measurement technique, and explain the basic features of the experimental phase diagram.